Weldable alloy

ABSTRACT

An alloy containing 20-23% chromium, 17-20% iron, up to 2.5% cobalt, 7-10% molybdenum, up to 1% tungsten, up to 0.15% carbon, 0.2-1% silicon, up to 1% manganese and the balance nickel is greatly stabilized with respect to weld crack resistance by the addition of lanthanum in effective but small carefully controlled amounts to 0.08% in the presence of silicon as an essential minor ingredient.

This application is a division of my co-pending application Ser. No.710,577, filed Aug. 2, 1976, U.S. Pat. No. 4,095,976 which in turn was acontinuation-in-part of my co-pending application Ser. No. 644,843,filed Dec. 29, 1975, abandoned which was in turn a continuation-in-partof application Ser. No. 99,738, filed Dec. 21, 1970, abandoned.

This invention relates to alloys, and more particularly to a solidsolution type alloy of the Ni-Cr-Fe-Mo class which is economicallyattractive, and versatile by virtue of a combination of oxidationresistance and resistance to development of cracks in the heat affectedzones produced during welding. The alloys provided are, therefore,particularly suited for use in structural parts designed for exposure tohigh-temperature oxidizing atmospheres, such as those commonlyencountered in gas turbine engines.

Manufacturers are insisting that costs be reduced. Anything whichcontributes to the ease of manufacturing reduces cost; therefore, theimproved weldability of an alloy manifested in fewer heat affected zonecracks reduces cost.

Products which last longer because they resist deterioration byoxidation resistance and attack by the environment reduce costs becausereplacement costs per unit time are less. The alloy of the inventionsatisfies this need.

Products which resist cracking because of thermal or mechanical fatigueare cost effective because they require fewer repairs and last longer.Alloys with improved high temperature ductility and a reduced number ofsurface defects caused by oxidation have greater resistance to fatiguedamage. Alloys of this invention meet these criteria.

Commercial usage of the alloy of the invention is anticipated in thoseareas in which MULTIMET®, RA 333, and HASTELLOY® alloy X are now used.The new alloy should extend the range of applicability by permitting thealloy to be used in more severely oxidizing conditions than prior artalloys.

Other possible applications are automotive thermal reactors, reformertubing and furnace hardware.

U.S. Pat. No. 3,304,176 to Wlodek shows that the addition of minoramounts of lanthanum is generally more beneficial in solid solution typenickel base alloys than in "precipitation strengthened" types. However,said patent does not teach the close control on the amount of lanthanumwhich is necessary for optimum weldability nor the essentiality ofsilicon and the optimum range of same to be used with lanthanum in thisconnection.

U.S. Pat. No. 3,383,206 also to Wlodek and a continuation-in-part ofU.S. Pat. No. 3,304,176 speaks of an alloy consisting essentially of, byweight 20-23 Cr, 8-10 Mo, 17-20 Fe, up to 0.15 C, 0-2 W, 0.05 to lessthan 0.3 La, 0.5-6 of the spinel forming elements selected from thegroup Co and Mn, the Co when selected being in the range of 1-3% and theMn when selected being in the range of 0.5-3% with the balance nickeland incidental impurities.

In U.S. Pat. No. 3,383,206 Wlodek taught that (1) manganese and cobaltin controlled amounts only up to 3 weight % each improved the oxidationresistance of the base alloy of 3,304,176 which contained 0.05 to 0.3w/o La, (2) either cobalt or manganese in an amount greater than 3 w/ocaused hot shortness in the alloy (Column 4, lines 72-75 and column 5,line 1); (3) tungsten and molybdenum are distinct and separate elementsand not substitutional as evidenced by the separate and distinct rangesof 0-2 W and 8-10 Mo rather than an inclusive range of 8-12 W+Mo, (4)the preferred range of La was 0.1-0.3 w/o and (5) an alloy containing0.17 w/o La particularly represented the alloy of his invention.

Wlodek did not teach any criticality of any alloying element withrespect to weldability and specifically he did not teach any criticalityof lanthanum content with respect to silicon content. Further, there isno evidence to suggest that he even suspected a criticality of La vs.weldability; he did not recognize that a problem existed.

Three heats of material of the same nominal compositions and within theWlodek patent range with the exception of lanthanum content weresubjected to weld tests. Two types of tests were used. The firstconsisted of welding a circular piece of the alloy to be tested to aheavy (about 1-inch thick) backup plate which had a hole in the centerof it which was slightly smaller than the test piece. A circular fusionweld bead about 2 inches in diameter was made with controlled weldingparameters. If a piece thus tested shows no evidence of cracking; theentire assembly is often aged to cause precipitation of second phasesthat tend to decrease ductility, impart additional stresses, and causecracking. If the sheet being tested does not crack, it is deemed to havegood restraint weldability. The second type of test--by far a lessstringent test because of the lower stresses involved--consisted ofsimply fusion welding a circle in a sheet of material clamped--notwelded--to a backup plate.

Alloy H, containing no La, was tested according to the first procedurebut without the aging. This material cracked in the weld. Alloy Icontaining 0.06 w/o La was also treated according to the first procedureincluding the post-weld aging. No cracks were found either before orafter aging with the exception of small crater cracks which are almostimpossible to eliminate at the finish point of the weld bead withoutusing filler rod.

Alloy J, containing 0.17 w/o La and a preferred composition of Wlodekwas tested according to the less demanding second procedure but withsimilar welding parameters. Gross cracking occurred.

Thus, it is shown that about 0.06 w/o La is beneficial to theweldability of this base material while as much as 0.17 w/o La, thepreferred amount in Wlodek, is detrimental.

While an analogous criticality of lanthanum apparently exists inWlodek's and related systems with regard to weldability as exists in thealloys of copending Ser. No. 644,843 invention, all evidence indicatesthat Wlodek was not aware of this criticality. In fact, alloy J of thisexperiment is from the same heat of material as Example 1 (Table II ofWlodek U.S. Pat. No. 3,383,206) of Wlodek, it is well within thepreferred composition range (Column 2, lines 3-5) of Wlodek, but had theworst weldability of those tested in the experiment. Alloy I, which hadthe best crack resistance contained near the optimum amount of lanthanumfor the alloys of the invention of Ser. No. 644,843, but was barelywithin the broad range of lanthanum content of Wlodek's system. HadWlodek recognized that a small but effective amount to less than 0.08w/o La was an optimum range for La, he certainly would have bracketedthis critical range in his claims and would have appropriately noted itin his specification.

One aspect which was noted, is that base compositions containinglanthanum contents above 0.1 w/o had marginal forgeability, whereasthose heats with lower than 0.1 w/o La forged well. Three of the seveningots of alloys of J, MM and NN failed completely during forgingwhereas 6 of 6 ingots of alloys I, KK and LL forged without difficultyand with excellent recovery.

    __________________________________________________________________________    EXPERIMENTAL ALLOYS                                                           CHEMICAL COMPOSITIONS, WEIGHT PERCENT                                         Alloy    Al Cr W  Fe C  Si Co Ni Mn Mo La                                     __________________________________________________________________________    X5-4791                                                                             H  -- 21.64                                                                            0.63                                                                             17.91                                                                            0.07                                                                             0.75                                                                             1.78                                                                             Bal.                                                                             0.65                                                                             9.0                                                                              --                                     66-527                                                                             *I  0.19                                                                             22.44                                                                            0.21                                                                             18.60                                                                            0.04                                                                             0.75                                                                             1.86                                                                             Bal.                                                                             0.90                                                                             8.96                                                                             0.06                                         J  -- 21.78                                                                            0.64                                                                             18.50                                                                            0.09                                                                             0.79                                                                             1.91                                                                             Bal.                                                                             1.0                                                                              9.16                                                                             0.17                                   66509                                                                              *KK -- 22.53                                                                            0.39                                                                             19.15                                                                            0.12                                                                             0.95                                                                             2.40                                                                             Bal.                                                                             0.85                                                                             7.62                                                                             0.03                                   66510                                                                              *LL -- 22.40                                                                            0.44                                                                             19.24                                                                            0.07                                                                             0.79                                                                             2.54                                                                             Bal.                                                                             0.89                                                                             7.75                                                                             0.03                                         MM 0.18                                                                             22.60                                                                            0.43                                                                             18.70                                                                            0.09                                                                             1.03                                                                             1.90                                                                             Bal.                                                                             1.08                                                                             8.57                                                                             0.13 av.                               66-523                                                                              NN 0.21                                                                             22.46                                                                            0.42                                                                             18.90                                                                            0.07                                                                             0.91                                                                             1.90                                                                             Bal.                                                                             0.94                                                                             8.57                                                                             0.13 av.                               X6-4568                                                                             OO -- 21.68                                                                            0.51                                                                             18.31                                                                            0.06                                                                             0.74                                                                             1.75                                                                             Bal.                                                                             0.53                                                                             9.15                                                                             --                                     __________________________________________________________________________     *Alloys of Invention                                                     

From this it is apparent that the two systems are different in the basicproperties of hot workability and weldability and therefore different inkind rather then degree.

One of the primary objects of the invention is to provide an alloy thatis weldable, oxidation resistant, stable, "but not unduly expensive."

Average electron vacancies (or Phacomp or simply N_(v)) calculations arean accepted method of predicting the formation of the detrimentalformation of topologically close packed phases (TCP).

Three articles (W. J. Boesch and J. S. Slaney, "Metals Progress" July1964, Vol. 86, No. 1, pp. 109-111; L. R. Woodyatt et al., TMS, April1966, Vol. 236, pp. 519-527; and H. J. Murphy et al., InternationalSymposium on Structural Stability in Superalloys, Sepetember 4-6, 1968,Vol. 1, pp. 47-61) review the concepts of N_(v) calculations adequately.

Generally, the lower the average electron vacancy concentration, thelower the likelihood of TCP formation. Increased amounts of elementssuch as Co, Fe, W, Mo and Si in lieu of Ni would increase the averagevacancy concentration. Conversely a decrease in the amounts of elementslike Mo would decrease the average vacancy concentration.

In order to obtain the optimum in oxidation resistance a minimum ofabout 0.2 w/o and preferably between 0.3-1 w/o Si is required in thealloy of this system. Because of its large N_(v) coefficient of 6.66 andthe difference in atomic weights of Si and Mo, only 0.5 w/o Si has thesame effect on the average electron vacancy concentration as 2.5 w/o Mo;Silicon has about 5 times the effect on N_(v) as Mo.

Tungsten and molybdenum are not substitutional in the alloys of theinvention even though many investigators have interchanged themsuccessfully in some instances. Wlodek, as noted, recognized this factwhen he established separate and distinct ranges for these elements forhis alloys. With respect to electron vacancy concentration and agedductility, Mo has twice the effect of raising electron vacancyconcentration as does W.

The objects and advantages of the present invention are achieved bysimultaneously controlling the silicon content of thesolution-strengthened alloy between about 0.2 and about 1%, andpreferably between about 0.3 and about 1% by weight, and the lanthanumcontent at a very small but effective level up to about 0.08%, andpreferably between about 0.02 and about 0.07% by weight. The majoringredients of my modified solution-strengthened nickel base alloy are(with amounts given in weight percent): about 20-23% Cr; about 17-20%iron; up to 2.5% Co; about 7-10% molybdenum; up to 1% W; effective to0.15% carbon, about 0.2-1% silicon, the balance nickel and incidentalimpurities. To improve strength in castings the preferred carbon rangeis 0.1-0.6.

Other common alloying ingredients such as zirconium, titanium, tantalum,columbium or hafnium may be present in small amounts in the presentinvention, they will be restricted to less than about 1 or 2% by weightof the alloy. Traces or fractional percentages of other elements such asboron and nitrogen are sometimes added for extraneous reasons as isknown in the metallurgical arts, but again, these play no significantrole in the present invention. For example, calcium, magnesium or otherdeoxidants are sometimes added during the preparation of a melt, butusually only traces or fractional percentages are retained as a residualpart of the finished alloy.

Aluminum in addition to being an excellent deoxidant is also believed tobe beneficial to oxidation resistance in small amounts up to about 0.5%.

Any of the standard methods conventionally used in manufacturingsuperalloys can be used in preparing the alloys of this invention, suchas air induction melting, vacuum melting, etc. Other than the normalcare required in analyzing constituents, intermediate combinations andfinished melts to insure attainment of the desired composition, the mostcritical step in the process is the addition of the lanthanum. In spiteof the small amounts of lanthanum required in the present invention, thedesired benefits therefrom can be obtained even if it is added in crudemixtures with other rare earth metals. Concentrated forms of lanthanumare preferably used in the present invention so that the lanthanumexceeds the total of all the other rare earths. One of the mostconvenient forms for use in the present invention is that of a prealloyor master alloy of a refined lanthanum source with nickel, cobalt and/orsilicon.

In any case, regardless of the form in which the lanthanum is added, itis normally convenient and efficient to add same during the late stagesof preparation thereof.

EXAMPLE I

To illustrate that the superb oxidation resistance demonstrated byWlodek is not impaired by utilizing lower lanthanum contents to achievesuperior hot forgeability and weldability an oxidation test wasperformed.

Six samples each of alloys I, NN and OO were exposed fifty hours at2200° F. Results show the oxidation resistance of the low lanthanumalloy I to be essentially equivalent to its high lanthanum counterpartNN and vastly superior to the base alloy OO without La.

    ______________________________________                                                Oxidation Test                                                                50 hours at 2200° F.                                           Alloy     weight change mg/cm.sup.2                                           ______________________________________                                        I         +1.9 ave       1 sigma 0.08                                         NN        +1.4 ave       1 sigma 0.09                                         OO        -56.9          1 sigma 13.9                                         ______________________________________                                    

Although specific embodiments of the present invention have beendescribed in connection with the above illustrative Example, it shouldbe understood that various other modifications can be made by thosehaving ordinary skills in the metallurgical arts without departing fromthe spirit of the invention taught herein. Therefore, the scope of thisinvention should be measured solely by the appended claims.

I claim:
 1. A weldable and oxidation resistant alloy consistingessentially by weight of:about 20--23% chomium about 17--20% iron up to1% tungsten about 7--10% molybdenum about 0.05--0.6% carbon about0.2--1% silicon up to 2.5% cobalt up to about 1% manganese an effectiveamount of lanthanum to provide weldability and high temperatureoxidation resistance to about 0.03% and in a sufficiently concentratedform that the total amount of all other rare earth metals is less thanthe amount of lanthanum, and the balance nickel, and incidentalimpurities.
 2. A weldable and oxidation resistant wrought alloy asclaimed in claim 1 containing 0.1 to 0.15% carbon.
 3. A weldable andoxidation resistant alloy with ranges in conformance with commercialmelting capability consisting essentially in weight percent ofabout:chromium--22 iron--18.5 tungsten--0.5 molybdenum--9 carbon--0.08silicon--0.5 cobalt--1.5 manganese--0.8 lanthanum in effective amountsto provide weldability and high temperature oxidation resistance up to0.03% and provided the total amount of rare earth elements other thanlanthanum is minimized and maintained below the lanthanum content andthe balance nickel and incidental impurities.
 4. An oxidation resistantwelded article made from an alloy consisting essentially of:about20--23% chromium about 17--20% iron up to 1% tungsten about 7--10%molybdenum about 0.05--0.6% carbon about 0.2--1% silicon up to 2.5%cobalt up to about 1% manganese an effective amount of lanthanum toprovide weldability and high temperature oxidation resistance to about0.03% and in a sufficiently concentrated form that the total amount ofall other rare earth metals is less than the amount of lanthanum and thebalance nickel and incidental impurities.
 5. An oxidation resistantwelded wrought article as claimed in claim 4 containing 0.1 to 0.15%carbon.
 6. An oxidation resistant welded article made from an alloy withranges in conformance with commercial melting capability consistingessentially of:chromium--22 iron--18.5 tungsten--0.5 molybdenum--9carbon--0.08 silicon--0.5 cobalt--1.5 manganese--0.8 lanthanum ineffective amounts to provide weldability and high temperature oxidationresistance up to 0.03% and provided the total amount of rare earthelements other than lanthanum is minimized and maintained below thelanthanum content and the balance nickel and incidental impurities.